{ Russian Olympiad. 2 stage.
  Problemset: http://agul-problems1.googlecode.com/svn/trunk/2010-2011/roi/stage2/roi_2stage.pdf
  Problem 5
  Points: 100
}
{$APPTYPE CONSOLE}
var s:int64; i,n:integer;
begin
reset(input,'input.txt');
rewrite(output,'output.txt');
s:=0;
readln(n);
s:=(n*(n+1)) div 2;
writeln(s);
close(input);
close(output);
end.